Version 3.20.4 for Hyrax 1.16.0

News for version 3.20.4

Fixed memory leaks in the libxml2-based DMR and DDX parsers.

Moved more toward C++-11. On CentOS 6 we now build using
c++0x while CentOS 7 uses c++11. This means that we can
use unique_ptr<> but not null_ptr, for example.

We have found a bug in the code that orders Dimensions,
Enumeration definitions and Groups in the DMR. Listing the
Groups last makes DMRs that reference Dimensions defined inside
Groups fail silently.